Carbon fibre composite materials are commonly employed in high cost/high performance applications where light-weight structures are required. For many applications, coupled with the high mechanical performance of carbon fibre composite materials is the unique visual appearance of carbon fibre composite laminates, in particular those using woven carbon fabrics. Such an aesthetic appearance of carbon fibres is highly desirable, and carbon fibre composite materials find many applications in high-value markets such as automotive components, luxury yachts and consumer electronics where the visual appearance of the carbon fibres as well as the technical performance of the composite material is important to the user/consumer. For these so-called “cosmetic” applications of carbon fibre composite materials, the visual aspect of the cured laminate needs to be of high quality and free from imperfections such as discontinuous fibres, particle contamination and surface effects, for example pin holes, fish eyes, blisters, etc.
Historically for the manufacture of cosmetic quality laminates, carbon fibre prepregs are used. The prepregs comprise at least one layer of carbon fibres and a curable thermosetting resin system, typically an epoxy resin. The thermosetting resin in these prepregs is typically cured using dicyandiamide-based catalysis. Dicyandiamide is added as a powder to the curable resin. The solubility of dicyandiamide in commonly employed resin chemistries is generally poor. Therefore, after cure, residual particles of dicyandiamide are frequently observed in areas of high resin content. To reduce this effect, the prepreg user is required to use low resin content prepregs and careful processing. These both introduce disadvantageous aspects to the component manufacture and do not fully mitigate the defects described.
Whilst it has been historically possible to employ liquid curing compounds such as imidazoles in order to achieve clear cured resins, such curing compounds have technical disadvantages such as short latency (leading to significantly reduced shelf life of the uncured prepreg), increased cost, and, depending on the chemistry used, often reduced thermal-mechanical properties and panel clarity, exhibited by excessive resin colour and opacity.
There is therefore currently a need in the art for a carbon fibre prepreg which includes a thermosetting resin system which not only provides a high quality processing, with a high latency of the curing system and a controlled curing, but also provides a combination of high quality thermo-mechanical properties and visual appearance.
WO-A-2012/113878 and WO-A-2012/113878 disclose liquid curing agents for thermosetting resins.